Click Modules in the list on the left. The Modules page lists resources, assignments, quizzes, pages, etc for your course. These items are arranged in modules, and put in a sequence to control the flow of the course. Modules are built to help you navigate the course content in an organized way.

You can click on the small triangles next to a module title to collapse and expand that module.

Modules 1.PNG

You can click on an item to access it. Once you've opened something, you will see Previous and Next buttons at the bottom to take you to the preceding or next item.

If you can not access a module it could be due to various reasons:

Some module content will have requirements you will need to complete before moving to the next part of the module or the new module.
A module may be locked and have prerequisites, things you have to access or complete, before you can view the content.
Modules can also be set to be available upon a certain date.

Tutorials:

Guide: How to view Modules